This invention relates to a paper feeding apparatus for feeding recording sheets in use with a copy machine, a facsimile, a printer, and so forth; and more particularly to a double sheet feeding preventing apparatus to prevent double feeding of recording sheets.
Conventionally, in printing apparatus such as a copy machine, a facsimile, a printer, and the like, a paper feeding apparatus, which can feed recording sheets one by one from the top or the bottom of stacked papers, such as a paper tray, a paper feeding cassette, and a paper stacker, and convey them to an image recording unit and the like, is installed.
In a document feeding unit of a copy machine or an image reading apparatus, a paper feeding unit, which can feed documents one by one from the top or the bottom of stacked documents such as a paper feeding tray, and convey them to an exposure unit on a document platen glass, is installed. Generally speaking, in this kind of paper feeding apparatus to feed recording sheets, disclosed in Japanese Patent Publication Open to Public Inspection Nos. 105834/1987 and 57447/1988, a feeding roller and a separating roller are installed which come into contact with each other ahead of a delivery roller. A sheet conveyed by the delivery roller passes through between the feeding roller and the separating roller. While the sheet passes through between the two rollers, it is separated from other sheets due to the function of the two rollers and double feeding is positively prevented.
Referring to the example shown in FIG. 6, the paper feeding apparatus mentioned above will be explained as follows. As shown in the drawing, in a case of an ordinary type of paper feeding apparatus, a delivery roller 1 is located with pressure on papers P piled up in a paper tray, and the paper P is conveyed towards the separating roller 3 by the rotation of the delivery roller 1. In this paper feeding apparatus, a paper separating mechanism consisting of the feeding roller 2 and the separating roller 3, which face each other, is located ahead of the delivery roller 1. When a plurality of papers P have been conveyed, in other words double feeding has occurred, only the top sheet passes through and the others are blocked or returned due to the function of the mechanism.
This type of mechanism is usually called 'a friction roller paper separating mechanism' and is in practical use in many paper feeding apparatuses.
In the paper separating mechanism shown in FIG. 6, the feeding roller 2 is driven through the gears Z1 and Z2 by a drive source not shown here, together with the shaft 4. The separating roller 3 is supported by the shaft 5 to which the gear 7 is fixed, and the shaft 5 is supported by a frame which is not shown in the drawing. The separating roller 3 can rotate freely because of the structure mentioned above. This frame is pressed or drawn by a spring located between the frame and the fixed portion of the copy machine frame in order to press the separating roller 3 to the feeding roller 2.
The gear Z7 is engaged with the intermediate gear Z6 which is engaged with the gear Z5 fixed to the torque limiter shaft 7 equipped with the torque limiter 6. The gear Z4 is mounted at the opposite end of the torque limiter shaft 7 and connected to the gear ZZ through the intermediate double gears Z3B and Z3A. Drive power is transmitted from the power source to the torque limiter shaft 7 by the gear train through the gears Z1 and Z2.
In the process of paper feeding by the paper feeding apparatus, first of all the upper surface of papers P is lifted up and the delivery roller 1 and the feeding roller 2 start to rotate according to a paper feeding signal. The delivery roller 1 which is pressed to the upper surface of papers with the prescribed pressure, conveys a paper P to the nip position between the feeding roller 2 and the separating roller 3. After that, the delivery roller is released from the paper surface.
The separating roller 3 is driven in the opposing direction of the feeding direction of the paper P through the above-mentioned gear train, Z2, Z3, Z4, and Z5, and the torque limiter 6. The separating roller 3 comes into contact with the feeding roller 2 with pressure by the function of the frame spring connected with the shaft of the separating roller, and the spring roller 3 rotates against the resistance created by the torque limiter 6.
When the separating roller 3 directly comes into contact with the feeding roller 2, in other words when there is no paper at the nip position or a sheet of paper P is conveyed into the nip position, the separating roller 3 follows the feeding roller 2 because the rolling friction caused between two rollers is more than enough to exceed the rotation resistance of the separating roller 3. But when more than 2 sheets are conveyed into the nip position, the rotation resistance of the separating roller is bigger than the friction between the sheets and the separating roller is reversed and pushes back the lower sheet to prevent double feeding. In FIG. 6, the continuous lines with arrow marks drawn on each rotation member show the rotation directions when sheet papers are not double fed; and the dashed lines with arrow marks show the rotation directions when the sheets are double fed.
However, in conventional paper separating mechanism, the torque limiter is rotated against the rotation resistance to the inner shaft by the power source through the gear train as explained above. Therefore, the gear train and the power transmitting shafts are necessary. Accordingly, space to locate them in the apparatus is required. As a result, the layout of other parts is restricted in design because of the above-mentioned drive unit. Furthermore, it is also an obstacle in the way of making the apparatus compact. Since this mechanism needs torque to drive the separating roller reversely to prevent double feeding of sheets, the power source is burdened with a heavy load at each time. The conventional mechanism requires a number of drive units, such as the gear train and other drive transmitting units. It results in increased cost of parts and increased labor to assemble and maintain the apparatus.